In wireless communications control channels are used to transfer control data from one node to another. Within 3GPP (3rd Generation Partnership Project) different control channels have already been specified.
Downlink transmissions in 3GPP LTE (Long Term Evolution) or in E-UTRAN (evolved Universal Terrestrial Radio Access Network) are based on Orthogonal Frequency Division Multiplex (OFDM). The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element (RE) corresponds to one OFDM subcarrier during one OFDM symbol interval.
In the time domain, with respect to LTE, transmissions are structured into frames and subframes. Each frame of length Tf=10 ms consists of ten equally-sized subframes of length Tsubframe=1 ms. Each subframe, in turn, consists of two equally-sized slots of length Tslot=0.5 ms. In frequency domain, the smallest addressable unit is a Resource block (RBs), which consists of 12 contiguous subcarriers during one slot.
In LTE data transmissions to/from a receiving node such as a User Equipment (UE) or a Relay Node (RN) are under strict control of the scheduler located in a control node such as an eNodeB (eNB) or a relay node (RN). Control signaling or control data is scheduled on a control channel and the control data is sent on the control channel from the control node to the receiving node (e.g. UE) to inform the receiving node about the scheduling decisions.
Downlink scheduling assignments and uplink scheduling grants are examples of control data transmitted via a (or as a) control channel. Downlink scheduling assignments are used to indicate to a UE that it should receive data from an eNB (or RN). A downlink scheduling assignment may assign certain resources for the transmission in the downlink, i.e. from a control node (e.g. a base station, an eNB or a RN) to a receiving node (e.g. a terminal or a UE). Uplink scheduling grants inform the receiving node (UE) that it should transmit in the uplink, e.g. a couple of subframes later. An uplink scheduling grant may grant certain resources for a transmission in the uplink, i.e. from a receiving node (e.g. a terminal or a UE) to a control node (e.g. a base station).
As an example, the downlink scheduling assignments (and uplink scheduling grants) point to a set of RBs or resource blocks groups (RBGs) in the frequency domain and refer to one subframe in the time domain, i.e., the assignments/grants may operate on pairs of resource block in the time domain.
Resource allocation can be done using different types of allocation. Type 0 allows allocating contiguous or non-contiguous resources in units of RBGs. Type 1 allows allocating non-contiguous RBs from a restricted set of RBs. Type 1 allows allocating one block of contiguous RBs. An example of resource allocation type 0 with RBG size of 3 RBs is shown in FIG. 2. In this example, the slopingly hatched parts in each frequency domain block 0 to 8 contain control data, whereas the horizontally hatched parts contain payload data. The depicted subframe is divided into a first slot and a second slot. E.G. in 3GPP TS 36.213 V8.8.0, section 7.1.6 three types of resource allocation are discussed.
Relaying is considered for LTE-Advanced (LTE release-10) as a tool to improve e.g. the coverage of high data rates, group mobility, temporary network deployment, the cell-edge throughput and/or to provide coverage in new areas. The relay node (RN) is wirelessly connected to the radio-access network via a donor cell controlled by a donor eNodeB (eNB). The RN transmits data to/from UEs controlled by the RN using the same air interface as an eNB, i.e. from a UE perspective there is no difference between cells controlled by a RN and an eNB.
LTE-Advanced will support a new control channel, the R-PDCCH. An R-PDCCH carrying downlink assignment occurs in the first slot and it occupies OFDM symbols numbered 3, 4, 5, and 6. An R-PDCCH carrying uplink grants is transmitted in the second slot of a subframe; under certain timing conditions the last OFDM symbol of the second slot cannot be used.
Multiple R-PDCCHs can be transmitted and similar to the LTE 3GPP Release 8 PDCCH the concept of a search space is applied: a search space is a set of locations in the time-frequency grid, where the relay node expects an R-PDCCH transmission. The Release 8 control region (i.e. the region, wherein the search space is determined) spans the whole freq. domain and the search space is determined taking all RBs into account. The R-PDCCH control region will typically not occupy the full system bandwidth so that the remaining resources can be used for transmission of data to UE and/or RNs. The region is configured by the eNB using the same resource allocation types as used for the allocation of Release 8 PDSCH (Physical Downlink Shared Channel). In general a search space is space in a transmission resource in which a receiving node (e.g. Relay node or UE) expects control data.
A further description of the Release-8 PDCCH can be found in 3GPP 36.213 V8.8.0 section 9.1 “UE procedures for determining physical downlink control channel assignment.” The PDCCH control region is defined by the value transmitted in PCFICH, hence it spans 1 to 4 OFDM symbols. The PDCCH search space is a UE-specific subset of REs within the control region.
The PDCCH control region is defined by the value transmitted in PCFICH (Physical Control Format Indicator Channel), hence it spans 1 to 4 OFDM symbols. The PDCCH search space is a UE-specific subset of REs within the control region.
The different resource allocation types have been design to minimize signaling load on the PDCCH and at the same time maximizing flexibility of PDSCH resource allocation. As a consequence, the three different types represent a trade-off between those two aspects.
The resource allocation is currently restricted to using type 0, 1 or 2. For instance, with resource allocation type 0 and a RBG size of 3, half of the R-PDCCH candidate positions of aggregation level 2 are not suitable for frequency selective scheduling. It is therefore desirable to further optimize the set up of a search space.
The R-PDCCH is one example of a control channel to which the subsequently presented configurations can be applied. The subsequently discussed configurations may also be applied to an ePDCCH (enhanced Physical Downlink Control Channel), which is another example of a control channel on which it is subsequently focused. The general concept may be applied to further control channels besides the ones explicitly mentioned.